Broken EIGRP in VPLS Context

Introduction

Commonly, Enterprise customers turn to carriers for providing WAN connectivity between remote offices. Today, most carriers today offer an emulated Layer 2 services on top of their MPLS cloud--a common carrier implementation being Virtual Private LAN Service ( VPLS ). While the details of the carrier implementation are outside the scope of this post, the interested reader may refer to RFCs listed at the end of this post for additional information.

As it relates to emulated Layer 2 WAN services, often Enterprise customers treat the WAN as an extension to the LAN. While this behavior is encouraged by the fact that you have one Layer 2 broadcast domain, it is folly. Particularly, many customers run IGPs ( EIGRP, OSPF, RIP, etc.) over this emulated Layer 2 service (I've even seen customers run STP!!). In the majority of cases, there are very few problems and the IGPs behave accordingly. However, in some instances, due to the service implementation of the provider or the software/hardware configuration of the customer, things go awry and IGPs behave in unexpected ways.

Background

In this post, I'll talk about an EIGRP issue I've seen in my environment, how it relates to L2VPN service implementation, and the work around I used to restore service.

For the purpose of this post, remote sites have two EIGRP speaking routers--RO-XA and RO-XB. Likewise datacenters have DC-XA and DC-XB. All the A-routers connect to VPLS provider A, likewise all B-routers connect to VPLS provider B. All routers are in single EIGRP domain.

Problem Description and Symptoms

While turning up one of our Regional Offices it was noticed that the EIGRP adjacency with multiple Regional Offices were flapping. 

RO-1B#show ip eigrp neighbors gi0/1

EIGRP-IPv4 Neighbors for AS(10)

H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq

                                                   (sec)         (ms)       Cnt Num

4   10.0.248.69             Gi0/1                    11 00:00:22    1  5000  1  0

6   10.0.248.72             Gi0/1                    12 00:00:53    1  5000  1  0

5   10.0.248.67             Gi0/1                    10 00:00:59    1  5000  1  0

3   10.0.248.76             Gi0/1                    13 14:05:32   30   180  0  212

2   10.0.248.65             Gi0/1                    11 14:05:32   20   144  0  646

RO-1B#

Looking in the logs, confirms how often the adjacency flaps. Since we will focus on DC-1B ( 10.0.248.69) below we see the flaps occurring regularly:

RO-1B#show clock

14:03:50.406 EDT Thu Oct 23 2014

RO-1B#show log | in Oct.*23 14.*.248.69

Oct 23 14:00:51.022: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is down: retry limit exceeded

Oct 23 14:00:51.826: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is up: new adjacency

Oct 23 14:02:11.338: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is down: retry limit exceeded

Oct 23 14:02:13.538: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is up: new adjacency

Oct 23 14:03:33.050: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is down: retry limit exceeded

Oct 23 14:03:34.866: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.0.248.69 (GigabitEthernet0/1) is up: new adjacency

We noted that although EIGRP adjacencies were flapping, there were no issues with unicast traffic or other broadcast type traffic. Indeed, between RO-1B and DC-1B ARP packets flowed freely. ICMP test ( all sizes) came back with no loss and latency within spec.

Buried in all this, we found another curiosity which is a focus of this post. We found that in the presence of flapping EIGRP adjacency, some adjacencies were half-formed:

• RO-1B would have stable adjacency with DC-2B
• RO-1B would have flapping adjacency with other Regional Offices
• Other Regional Offices wouldn't even see or start a neighbor adjacency with RO-1B, 

High-Level Troubleshooting Efforts:

So we had flapping adjacencies, one way adjacencies, and suboptimal routing, and apparent disregard for the EIGRP split-horizon/poison reverse rule. Our main concern was trying to understand the adjacency flaps--the other things were taken as side effects but are interesting enough that they warrant discussion.

First, lets clear up the preliminaries. We did the following, not necessarily in order:

• Removed any policy-map configuration on th R01-B interface
• Replaced R01-B with a different model/platform
• Replaced physical wiring to the carriers CPE
• Performed extended pings with sizes from 100 Bytes to 1250 Bytes (could be fragmentation issue)
• Verified we can ping 224.0.0.10, the EIGRP multicast address

We had the carrier check the following for the devices in their network on the path RO-1B to their PE ( so called Access network in carrier parlance):

• Fiber light levels from their CPE to PE were at as-built levels and tended not to fluctuate
• Perform RFC2544 test for frame loss
• Verify CPE hand off port had no errors/drops/oversubscription
• Verify traffic was not being dropped due to BUM filtering. BUM filters, or broadcast/unknown unicast/multicast filters are a standard way to limit the amount of flooding occurs over an emulated Layer 2 service
• Verify service configuration on their equipment

A special note about BUM filtering. Too much broadcast/unknown unicast/multicast traffic between customer sites can easily cause oversubscription/resources issues in the carrier network. This condition is too easily created by a customer using a bridge-like service due to customer misconfiguration. This is because the carrier PE must replicate ( if they use ingress replication) each BUM packet and send to the other PEs in the L2VPN--which comes at a cost. The interested reader may refer to RFC5501 ( Section 4 ) AND RFC4761 ( Section 4 ) for a detailed treatment of this problem in a carrier environment.

Finding a work around:

As mentioned previously, there were various canaries in the coal mine and were key clues to finding a work around:

• We could ping the EIGRP multicast address (224.0.0.10) and did receive replies from some of the EIGRP speakers. For each non-replying router, we could still reach the router via unicast ping
• While EIGRP sessions were flapping, unicast traffic worked just fine

The work around was to use EIGRP unicast mode ( example here)-- instead of EIGRP speakers using multicast to discover neighbors, we explicitly configure the neighbors. All routers then send EIGRP packets directly to configured neighbors.

As outlined in Cisco FAQs, this feature isn't without is caveats. Namely, once you configure unicast neighbors on an interface, all multicast packet processing stops. It's all or nothing.

Conclusion:

While I've seen this issue in an EIGRP context, there's no good reason why other multicast-based IGPs or protocols wouldn't be impacted. This includes protocols that only use multicast for a subset of messages/functionality. Their behaviors under a carrier-multicast outage will differ. In our case, we had suboptical routing ( using Remote sites with lower bandwidth profile as transit for site-to-site flows), equipment failures/issues due to reconvergence, and even sites being isolated from the network.

Essentially, the problem was with the carrier's ability to deal with multicast/broadcast traffic. As we've learned from RFC7117 there are different approaches to the BUM problem in a VPLS carrier environment. Some implementations use ingress replication exclusively ( Junos PDF) ans some use formal multicast distribution tree within the MPLS cloud ( Cisco http).

While I'll never really know the specifics of the problem within the carrier's network, I'm told it was a router software bug.

Cheers!

References:

Normative References

All Virtual Private LAN Services RFCs ( listing ); in particular RFC4761, RFC5501 and RFC7117

Informative References

RFC3031: Multiprotocol Label Switching Architecture ( http )

RFC4364: BGP/MPLS IP Virtual Private Networks (VPNS) ( http )

RFC3107: Carrying Label Information in BGP-4 ( http )

[EIGRP-Protocol] "Enhanced Interior Gateway Routing Protocol" Technology Whitepaper ( http )