Routed VLT vs VRRP

There were a lot of queries on how R-VLT can be used in place of VRRP and what are the advantages and disadvantages. Further to this I had a call with some of you and provided answers to all these queries. I am summarizing the details here. If there are further questions please email me.

R-VLT or peer-routing (as it is called in configuration guides) provide the ability to do the default gateway functionality similar to VRRP. Apart from that, one can run any IPv4 / IPv6 routing protocols in a VLT based core with R-VLT turned on. Please note this is a unique capability among vendors that offer MLAG type solutions.  For more information on benefits of R-VLT, please refer to ‘VLT Overview 2.0’ AND ‘VLT reference architecture 2.0” in sales edge.

Ø  R-VLT is recommended where larger # of L3 VLANs are required. VRRP has a limitation of 255 L3 VLANs, so if the customer requires more than 255 L3 VLANS please recommend R-VLT. Today we support 512 L3 VLANs in R-VLT and we are going to increase this number to 2,500 L3 VLANs in OS 9.7 (Klamath) release

Ø  There was a limitation when a peer-node goes down the other peer was not responding to ARP requests sent to default gateway address. This was fixed when “Proxy ARP” functionality for peer-VLT nodes was introduced in OS 9.3

Ø  VRRP-v4 and R-VLT can co-exist together. Certain set of VLANs can be using VRRP and other set can be using R-VLT. This is required during migration from VRRP to R-VLT and the customer is not willing to convert all VLANs to R-VLT in one shot

Ø  Caveat with R-VLT: In a VLT setup between switches A and B, where A’s IP is configured as default gateway, when both the nodes go down and only B comes up, the gateway functionality will not be available. R-VLT requires a handshake between A and B at least once. Hence in such cases the network will not have a gateway until the other node comes up. 

o    This is a corner case that can happen when both the nodes go down and only one comes up (power outage with other issues) or when software is upgraded on both VLT node (upgrade done one after other). The former will have other network issues like convergence etc. as there is a DC wide power outage and the later will likely be done during a maintenance window.

o    We are exploring ways to solve this issue in the future OS 9.x release, but there are no simpler ways of solving this issue. So unlikely to get fixed in the near future. 

For all the functionalities introduced in VLT since OS 9.2, please refer to corresponding DNL slides available on DNL playback site. For any other queries please reach out to me or ASK-NETWORKING-PLM alias.

Thanks & regards,

Shankar Vasudevan

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Product Manager | Dell Networking | Enterprise Solutions Group

office: +91-44-3920-8451, mobile: +91-9500018850

Chennai, India