Convergence Delay of a Routing Update For Static, RIP, OSPF, EIGRP, ISIS and BGP

Got some good docs for the maximum Convergence Delay of a Routing Update For Static, RIP, OSPF, EIGRP, ISIS and BGP.
Br/> Using the below described convergence points the theoretical maximum propagation delay of a routing update can be calculated from one VPN site to another VPN site (CE router to CE router).

- The convergence points are taken as T1_max, T2_max, T3_max, T4_max till T8_Max

- As per the convergence points the maximum theoretical convergence times are calculated for various protocols like RIPv2, EIGRP, OSPF, ISIS, BGP and static routes.

- The total convergence time taken for a static route is 25 seconds

- The maximum convergence time for RIP approximately 85 seconds for new routes, 25 seconds for serial links and 265 seconds in case of route flap.

- For EIGRP the convergence timer is much faster and is around 25 seconds.

- The convergence time increases slightly to 35 seconds in case of OSPF

- For ISIS the maximum theoretical convergence value is same as OSPF approximately 35 seconds.

- The convergence value increases to 85 seconds in case of BGP.


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Loop Free Alternate: IP Fast Reroute (FRR)

We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the Outages duration when the primary uplinks fail on a router.

When a link or a router fails, routing algorithms like ISIS or OSPF compute new routes that take into account the failure. The time taken for computation is called routing transition. Until the transition is complete and all routers are converged on a common view of the network, the connectivity between the source and destination pairs is interrupted. Routing transition depends on the numbers of routers present in that area. The bigger the area; more the time is. This may impact on revenue of the customer also. We can use the IPv4 Loop-Free Alternate Fast Reroute feature to reduce the routing transition time to less than 50 milliseconds using a pre-calculated alternate next hop. When a router is notified of a link failure, the router immediately switches over to the repair path to reduce traffic loss.

The repair path computation is done by the Intermediate System-to-Intermediate System (IS-IS) or OSPF and the resulting repair paths are sent to the Routing Information Base (RIB). The repair path installation is Forwarding Information Base(FIB).

Eigrp has it's own Loop Free Alternate.
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Which Routing Protocol To Select? OSPF Vs EIGRP

Most of the times, network administrators and planning guys look forward to understand which routing protocol are best to deploy for their network. The selection criteria between OSPF and EIGRP based on the below points:-

1. Between OSPF and EIGRP which one is the best to deploy and why?
2. Which protocol converges faster and highly available?
3. Which protocol uses fewer resources?
4. Which protocol is easier to deploy and operate?
5. Which protocol is easier to understand and configure?
6. Which protocol is easier to scale in large network?
7. Which protocol is more scalable and easy to adopt changes?

Every protocol has its own merits and de-merits. Between OSPF which is a link state routing protocol in its own area and becomes a distance vector routing protocol from one area to another and EIGRP is a distance vector routing protocol, In fact, its not a pure DV routing protocol.

Convergence speed depends on the number of routers and routes involved in the network. The more the number, less the convergence time. Convergence speed depends on various attributes defined in Fast Convergence Tools.

At broader level below are few points which can be used as high level to understand the protocol at its best:-

1. EIGRP uses metric based on bandwidth, delay, reliability, load and MTU whereas OSPF uses interface cost which is inversely proportional to bandwidth. EIGRP is considered as best in terms of selecting the path on different attributes.
2. EIGRP is proprietary to CISCO whereas OSPF is based on OPEN Standard.
3. EIGRP sends hop by hop query when feasible successor is not found whereas OSPF syncs its LSA(LSA1, LSA2, LSA3, LSA4 and LSA5) database whenever there is change in network topology. EIGRP is considered as best as it minimizes the routing information.
4. EIGRP is simpler to understand whereas OSPF has lot of things to understand. It depends how comfort you are to select out of these.
5. EIGRP does automatic summarization whereas OSPF doesn’t.
6. EIGRP does support both equal and unequal cost load sharing whereas OSPF doesn’t.
7. EIGRP limits the usage by 50% of the link bandwidth whereas OSPF does 100%.
8. EIGRP is faster to converge when it has feasible successor but OSPF doesn’t. Still there are lot of tools available to make OSPF better in terms of convergence.

Even after so many years, we can’t say which protocol is best over another. All it depends on your business requirement, understanding, behavior of deployed applications and network design. So decision is absolutely yours.


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What is Discard Route or NULL0 Route?

What is Discard Routes and how does it help to stop layer 3 routing loops? As we know the fact that loops are not only originates in layer2 network but could also originate in layer 3 networks. We have various mechanisms to prevent layer 3 routing loops with spit horizon and route poisoning.

Would like to understand; IS there any kind of other scenario where in routing loop could exist? See the below infographic

As depicted above, routing loop could occur whenever a summary route is announced towards the peer. This loop could be stopped by creating a manual discard route pointing towards null interface.

IP Route 10.2.0.0 255.255.0.0 null0

OSPF, IS-IS and EIGRP automatically creates discard route whenever summary is performed on the routers.


Below are the configurations for disabling discard routes:-
OSPF
router ospf 1
no discard route

EIGRP
interface serial0 ip summary-address eigrp 1 10.2.0.0 255.255.0.0 255


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CISCO EIGRP DUAL Algorithm

Enhanced Interior Gateway Routing Protocol (EIGRP) is a advanced distance vector routing protocol proprietary to CISCO. Highly valued for its ease of deployment and fast convergence, EIGRP is used in many large Enterprise networks. EIGRP maintains all of the advantages of distance-vector protocols and having good features set for selecting loop free paths too.

EIGRP scales effectively in a well-designed network and provides extremely quick convergence times with minimal network traffic. EIGRP advantages include:
• Consumes low network resources as compared to OSPF
• Transmits only partial updates not the full routing table
• Rapid convergence times for changes in the network topology

Deep dive to get better understanding of Cisco EIGRP DUAL Algorithm

What is Reported Distance?
As depicted in Figure 1, A has three directly connected neighbors to reach E. The distance reported by A’s neigbors to reach E is known as Reported Distance. So A has three reported distance from it’s neighbors as below mentioned:-
• B can reach E with a cost of 10
• C can reach E with a cost of 10
• D can reach E with a cost of 30

What is Feasible Distance?
The total cost of each link to reach from A to E is known as feasible distance. As depicted in figure 2, A has three feasible distance to reach E with below mentioned cost.
• A can reach to E via B with cost of 20
• A can reach to E via C with cost of 25
• A can reach to E via D with cost of 45
The best out of the three feasible distances will become the successor. So as per the above output, B is nominated as the successor.

How to select the feasible successor or loop free alternate? After selecting successor, DUAL will look for the Reported Distance of the neighbors and check which one satisfies the Feasibility Condition; Reported Distance < Feasible Distance. As per the above two outputs we can conclude that
• C can reach E with a cost of 10(Reported Distance), so C reported distance (10) < feasible distance (20). This path is a loop free.
• D can reach E with a cost of 30(Reported Distance), so D reported distance (30) > feasible distance (20). This doesn’t satisfies the condition of selecting feasible successor (Reported Distance < Feasible Distance). So DUAL will mark this path as loop.

So now A has marked B as its successor and C has its feasible successor. Let’s assume now link between A and B is down as per figure 3. Now EIGRP will examine the available paths to E and declared C as a successor (best path to reach E) which was earlier selected as feasible successor (Loop Free Path). Now EIGRP will again look for its feasible successor and as per the Feasibility Condition, Reported Distance from D is 30 which is greater than the Feasible Distance. So EIGRP will consider D as loop path and didn’t qualify it for the feasible successor.

Now assume, link between A and C is down as shown in Figure 4. Now EIGRP will look for loop free path but unfortunately, as shown in Figure 3, no loop free path is available. However, A has a neighbor D might have a loop free path to E. So A will put E into Active State and query to D, D sends an reply to its query that it has a loop free path available to E. Once A will receive the reply from D, it beings start using that path for E.

Read More About Remembering PE-CE EIGRP

Read More About Eigrp adjacency issues with TLV


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Remembering PE-CE EIGRP

1.The EIGRP configuration is identical to RIP.
2.The autonomous system aka AS number can be manually set under the EIGRP VRF routing process.
3. If same AS is used among PE routers
- Internal VPN routes of VPN will be learned as internal routes to other VPN.
- External routes will be seen as external to another VPN sites.
4. If different AS is used among PE routers
- Internal VPN routes of VPN will be learned as external routes to other VPN.
- External routes will be seen as external to another VPN sites.
5. BGP MED and extended communities are used to carry the AS number, route type and metric.
6. BGP Cost community plays an vital role in EIGRP. This is the only community which helps to select the route on the basics of cost not on administrative distance that's why iBGP route selects over the EIGRP route even EIGRP route has more AD number than iBGP.

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Eigrp adjacency issues with TLV

Approx 15 days back we changed the IOS of one of our PE 7200 router from 12.4(15)T to 12.4(22)T. After up gradation of IOS we faced weird problem of PE-CE eigrp. Every instant the eigrp neighbor comes up and goes to down. We collected the logs and forwarded to cisco. After few days cisco conforms that the IOS is having an eigrp bug in PE-CE. We changed the IOS & problem dissipate after up gradating to 12.4(11)T4.
Few days’ back one of my friend came up with the same problem but the ios used by them is other one. I don't know the name of the IOS. He was also facing the same PE-CE eigrp fluctuation. But as per him ios had not been changed or upgraded. He booked case in cisco and got reply from cisco.

Cisco Reply: when sending 10.x.x.x prefixes, the PE happens to combine them in such way, that resulting packet length is always under 1500;
- when 192.168.x.x are added, there appears a combination of TLVs, which results in length less than 1560, but more than 1500 thus jamming the update transmission.
Is this really true or false ? Still looking for relevant reply or solution.


regards
shivlu jain
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