Forwarding address (FA) in OSPF LSAs

In scenarios where we have multiple ABRs converting/translating same network from Type7 to Type5 LSA, then OSPF database need to act on only one of them. The other must withdraw its LSA. This is one of many loop avoidance techniques adopted in OSPF. The RFC 2328 says “if two routers, both reachable from one another, …

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BGP deterministic MED and always compare MED options

The BGP MED (multiple exit discriminator) is an optional non-transitive attribute. That means this value is always carried along with BGP update and can be dropped if the BGP process doesn’t understand it. Let me first write about my understanding on both commands bgp deterministic-med command compares MED variable when choosing routes advertised by different peers …

Update-source ‘interface’ in BGP

When we define a neighbor under BGP process, by default BGP uses the appropriate egress interface IP address to reach the neighbor. Once both the peers agree on the parameters the neighborship comes up. If we want to build the neighborship over a specific interface IP address rather than the egress interface IP address, then …

Destination NAT (ip nat outside source static x.x.x.x y.y.y.y)

We get a lot of examples on NAT configuration based on inside NAT aka source NAT but we rarely use outside NAT aka destination NAT. In this article we will discuss about a used case of destination NAT. Before we start our discussion on destination NAT (‘ip nat outside source static X Y’ command) and …

Default route (0.0.0.0/0) LSA Types in different OSPF areas

As we already know OSPF has different area types (normal, stub, totally stub, not-so-stubby/nssa and nssa totally stub) and the LSAs permitted inside them varies accordingly. In OSPF the default-route is treated differently. By default OSPF does not allow a default-route from any other domain to be advertised/redistributed into its domain. We need to configure …

VRRP vs Dell VLT Peer-routing

VLT (Virtual Link Trunking) is a Dell Networking proprietary protocol. This article describes the working functionality of VRRP and Peer-routing when implemented over VLT. For detail configuration I would suggest you to refer the Dell Networking Configuration guide for OS9 platforms. What is Dell Networking VLT (Virtual Link Trunking)? In simplified words, a VLT domain …

Route-Distinguisher & Route-Target

For MPLS VPN to work (end-to-end) we need Route-Distinguisher, Route-Target, Transport Labels and VPN labels. But these four entities are not dependent on each other rather without any one of them the MPLS VPN is not complete. In this article I will explain the significance of RD & RT only (I am not covering LDP …

MULTICAST – PIM SPARSE MODE

PIM takes care of providing a loop free path for multicast traffic by referring to the unicast routing table. In case of PIM Sparse mode, RP acts as the root of the shared tree. Everything needs to be sent to RP first and then shortest path can be calculated if applicable. For the multicast traffic …

DMVPN PHASE III – PART 2

PART-2 covers below mentioned details Understanding various NHRP request and resolution packets Role of ‘ip nhrp rediret’ on hub Role of ‘ip nhrp shortcut’ on spoke In my earlier post DMVPN Phase III PART 1 , we have already gone through the NHRP registration process between hub & spokes and spoke to spoke dynamic tunnel creation. …

DMVPN PHASE III – PART 1

PART-1 covers below details Hub and Spoke Configuration NHRP registration Flags in NHRP registration request EIGRP route summarization on HUB Spoke to Spoke tunnel Before starting our discussion on DMVPN Phase III, let’s discuss few pros and cons of DMVPN Phase II. DMVPN Phase II Pros: Hub and Spokes are configured as multipoint GRE tunnel …