Category Archives: Routing and Switching

DHCP Snooping and Option-82

Even though I always had a solid high level understanding of DHCP Snooping feature, I had no idea what happens ‘under the hood’. Until now. We’ve never been using this security enhancement (for whatever reason), but things are changing now. You’ll be surprised to find out that we’re deploying DHCP Snooping primarily to improve Cisco ISE Profiling. If you want to know more, read about Cisco IBNS 2.0 and Profiling Design Guide, specifically profiling using Device Sensor and Radius accounting. I might be able to cover this topic in detail sometime later (let me know if it’s of interest). Today, however, I’d like to concentrate on DHCP Snooping. Continue reading

Directed Broadcasts and IOS

I’ve been reading about directed broadcasts recently. According to RFC2644, directed broadcasts should not be forwarded by routers to the end hosts on the destination subnet. It wasn’t very clear if routers should drop directed broadcasts. If so, should these be dropped by intermediate router, or the one at the final destination? So, I decided to build a small lab and capture few packets. Continue reading

eBGP Loopbacks: Multihop vs Disable Connected Check

By default, BGP checks if its configured external (or eBGP) peers are directly connected. If not, BGP will not initialize for those peers (will stuck in Init). Respectively, this is true when destination’s IP address belongs to a loopback interface, even though it is configured on the directly connected router. Moreover, default TTL value for eBGP packets is 1 (that is why the check is required). There are two ways to change router’s default behavior – configure eBGP multihop or disable the check for directly connected peers. They both do same thing for loopback interfaces, yet the logic is different. Continue reading

EIGRP Offset Lists Logic

EIGRP Offset Lists allow to add a value (an offset) to the calculated integer metric (RD/FD) for a given prefix. How does this fit into the composite metric’s concept? We know EIGRP’s Update messages contain the following metric components – cumulative delay, least bandwidth, load and reliability; as well as non-metric components – hop count and MTU. So, where’s offset? Continue reading