;-)-repl.clj

user=> (def c [{:next-hop "192.168.1.1", :prefix "172.16.1.1/32", :route-status "*|e"}
  #_=>         {:next-hop "192.168.255.255", :prefix nil, :route-status "*>e"}]) 
#'user/c
user=> 

user=> (let [prev (atom nil)]
  #_=>   (map (fn [r]
  #_=>          (if (and @prev (not (:prefix r)))
  #_=>            (assoc r :prefix @prev)
  #_=>            (do (reset! prev (:prefix r))
  #_=>                r)))
  #_=>        c))
({:next-hop "192.168.1.1", :prefix "172.16.1.1/32", :route-status "*|e"}
 {:next-hop "192.168.255.255", :prefix "172.16.1.1/32", :route-status "*>e"})
user=> 

user=> (second
  #_=>   (reduce
  #_=>     (fn [[prev acc] v]
  #_=>       (if (and prev (not (:prefix v)))
  #_=>         [prev (conj acc (assoc v :prefix prev))]
  #_=>         [(:prefix v) (conj acc v)]))
  #_=>     [nil []]
  #_=>     c))
[{:next-hop "192.168.1.1", :prefix "172.16.1.1/32", :route-status "*|e"}
 {:next-hop "192.168.255.255", :prefix "172.16.1.1/32", :route-status "*>e"}]
user=> 

user=> (loop [prev nil
  #_=>        acc []
  #_=>        coll c]
  #_=>   (if-let [v (first coll)]
  #_=>     (if (and prev (not (:prefix v)))
  #_=>       (recur prev (conj acc (assoc v :prefix prev)) (rest coll))
  #_=>       (recur (:prefix v) (conj acc v) (rest coll)))
  #_=>     acc))
[{:next-hop "192.168.1.1", :prefix "172.16.1.1/32", :route-status "*|e"}
 {:next-hop "192.168.255.255", :prefix "172.16.1.1/32", :route-status "*>e"}]
user=> 

all.clj

(def c [{:next-hop "192.168.1.1", :prefix "172.16.1.1/32", :route-status "*|e"}
        {:next-hop "192.168.255.255", :prefix nil, :route-status "*>e"}])

; map with atom
(let [prev (atom nil)]
  (map (fn [r]
         (if (and @prev (not (:prefix r)))
           (assoc r :prefix @prev)
           (do (reset! prev (:prefix r))
               r)))
       c))

; reduce with compound accumulator
(second
  (reduce
    (fn [[prev acc] v]
      (if (and prev (not (:prefix v)))
        [prev (conj acc (assoc v :prefix prev))]
        [(:prefix v) (conj acc v)]))
    [nil []]
    c))

; loop/recur
(loop [prev nil
       acc []
       coll c]
  (if-let [v (first coll)]
    (if (and prev (not (:prefix v)))
      (recur prev (conj acc (assoc v :prefix prev)) (rest coll))
      (recur (:prefix v) (conj acc v) (rest coll)))
    acc))

show-ip-bgp-cisco-nexus.txt

*|e172.16.1.1/32      192.168.1.1                                    0 65535 i
*>e                   192.168.255.255                                0 65535 i